Police arrested the youths, including two schoolboys, from the spot.
The arrestees are Amir Hossain, 20, son of Khalilur Rahman of Koyerdara area, Avijit Halder Rinku, 24, son of Ranjit Halder of Sapura Pobapara area, and Mobarak Hossain, 18, son of Emajuddin, of Sahedbazar Masterpara area of the city.
Among them, Rinku is BBS first year student of Rajshahi College, Mobarak is a 10th grader of Technical Training Centre and Amir is a student of the same class of Nawhata High School.
Nibaran Chandra Barmon, officer-in-charge of Boalia Police Station, said the three youths attacked ASI Mainul Islam with knives while being searched on suspicion at Kadirganj of the city around 11:30, leaving assistant sub-inspector Mainul Islam injured.
Police recovered a foreign pistol, four rounds of bullet and a magazine from their possession, the OC added.